Previously, acrylic fiber strands have been widely known and put to practical use as raw fiber materials for manufacturing carbon fibers.
Acrylic fiber strands for manufacturing carbon fibers are manufactured from polymers containing at least 90% by weight or more, preferably 95% by weight or more, of acrylonitrile through the steps of spinning, stretching and drying.
As the spinning solvent, organic solvents such as dimethylformamide (DMF), dimethylsulfoxde (DMSO), dimethylacetamide (DMA), etc., and inorganic solvents such as zinc chloride, nitric acid, rhodanate, etc. are used. In particular, as the spinning solvent for acrylic fiber strands to be used for manufacturing carbon fibers, a zinc chloride-containing aqueous solution is preferred and can be used efficiently.
One important factor affecting the quality of the acrylic fiber strands for manufacturing carbon fibers is the problem of coalescence of filaments. The degree of coalescence varies from a state in which the filaments are lightly adhered to one another to a state in which two or several of the filaments are integrated with no boundary. Such coalescence of the filaments results in fiber breakage during the manufacturing carbon fibers, causing fluff on the strand, and lowering the strand strength, and accordingly causes problems in the stability of the manufacturing steps and the quality of the fiber products. Therefore, it is extremely important to manufacture coalescence-free acrylic fiber strands for manufacturing carbon fibers.
In general, in the wet spinning of acrylic fiber strands, the steps of spinning, solvent removal, post-stretching, drying, etc, are carried out, and in the case of acrylic fiber strands for manufacturing carbon fibers, the strength of the raw fiber materials often has a great influence on the strength of the resulting carbon fibers. Accordingly, the acrylic fiber strands for manufacturing carbon fibers are highly stretched during manufacture in most cases, whereby fiber products having high molecular orientation are generally obtained. Further, since the acrylic fiber strands for manufacturing carbon fibers are intermediate fibers in the course of manufacturing the carbon fibers, these do not always require the relaxation (shrinking) treatment which is generally applied to acrylic fiber strands for general use (e.g., fabrics), in a degree of about 30% after completion of drying for the purpose of improving the knot strength. This is because the relaxation treatment would cause the relaxation of molecular orientation, which is unfavorable for raw materials for manufacturing carbon fibers, which are required to have a high strength.
On these grounds, the running fiber strands are in most cases subjected to manufacturing steps under a tension which would not permit the relaxation of the molecular orientation, during the manufacturing steps of the fibers.
However, if acrylic fiber strands for manufacturing carbon fibers are manufactured under such conditions, the coalescence of the fibers frequently occurs, and the coalescence of the raw fiber materials causes breakage or combustion of the fiber strands during a preoxidation process, and further reducing the quality of the final carbon fiber products.
During manufacture of acrylic fiber strands for manufacturing carbon fibers, a system has been adopted for increasing productivity that includes arranging plural strands parallel to one another, passing them through manufacturing steps in the form of a sheet, and thereafter separating the sheet of strands into individual strands. In this case, the strands must not be entangled, so they can be easily separated in the last separation step. However, the filaments are often loosened and become entangled with the strands during the manufacturing steps, and as a result, separation becomes difficult and the strands often become fluffy.